Rotational Actuator of Motor Based on Carbon Nanotubes

ABSTRACT

A rotational actuator/motor based on rotation of a carbon nanotube is disclosed. The carbon nanotube is provided with a rotor plate attached to an outer wall, which moves relative to an inner wall of the nanotube. After deposit of a nanotube on a silicon chip substrate, the entire structure may be fabricated by lithography using selected techniques adapted from silicon manufacturing technology. The structures to be fabricated may comprise a multiwall carbon nanotube (MWNT), two in plane stators S 1 , S 2  and a gate stator S 3  buried beneath the substrate surface. The MWNT is suspended between two anchor pads and comprises a rotator attached to an outer wall and arranged to move in response to electromagnetic inputs. The substrate is etched away to allow the rotor to freely rotate. Rotation may be either in a reciprocal or fully rotatable manner.

CROSS-REFERENCE TO RELATED APPLICATIONS

This divisional patent application relates to and claims benefit ofProvisional Patent Application Ser. No. 60/488,485, filed Jul. 18, 2003,and U.S. patent application Ser. No. 10/891,615, filed Jul. 15, 2004,both of which are hereby incorporated by reference in their entireties.

STATEMENT OF GOVERNMENTAL SUPPORT

This invention was made during work supported by U.S. Department ofEnergy under Contract No. DE-AC03-76SF00098. The government has certainrights in this invention.

REFERENCE TO SEQUENCE LISTING OR COMPACT DISK

None.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the field of nanomaterials such ascarbon nanotubes and further to the field of molecular sizedelectromechanical devices.

2. Related Art

Nanostructures are of great interest not only for their basic scientificrichness, but also because they have the potential to revolutionizecritical technologies. The miniaturization of electronic devices overthe past century has profoundly affected human communication,computation, manufacturing and transportation systems. Truemolecular-scale electronic devices are now emerging that set the stagefor future integrated nanoelectronics (Ref 1).

Recently, there have been dramatic parallel advances in theminiaturization of mechanical and electromechanical devices (Ref 2).Commercial microelectromechanical systems now reach the submillimeter tomicrometer size scale, and there is intense interest in the creation ofnext-generation synthetic nanometer-scale electromechanical systems(Refs 3,4). Such a nanometer scale electromechanical system is describedbelow, demonstrating the construction and successful operation of afully synthetic nanoscale electromechanical actuator/motor incorporatinga rotatable metal plate, with a multi-walled carbon nanotube serving asthe key motion-enabling element.

Although devices have been made by scaling down existingmicroelectromechanical systems (MEMS), the workhorse methods andmaterials of MEMS technology are not universally well suited to thenanoscale. Ultra-small silicon-based systems fail to achieve desiredhigh-Q mechanical resonances owing to dominant surface effects andthermoelastic damping, and limitations in strength and flexibilitycompromise silicon-based high-performance actuator/motors (Refs 5, 6).On the other hand, the unusual mechanical and electronic properties ofcarbon (Ref 7) and boron-nitride (Ref 8) nanotubes (including favorableelastic modulus and tensile strength, high thermal and electricalconductivity, and low inter-shell friction of the atomically smoothsurfaces (Refs 9, 10) suggest that nanotubes may serve as importantNEMS-enabling materials if nanotubes can be engineered and modified tobe part of a higher order system, i.e. as active components in a movabledevice.

Cumings et al. U.S. 2002/0070426 A1 discloses a method for forming atelescoped multiwall carbon nanotube (“MWNT”). Such a telescopedmultiwall nanotube is shown in this publication to act as a linearbearing in an electromechanical system. That is, the walls of amultiwalled carbon nanotube are concentrically separated and are shownto telescope axially inwardly and outwardly. In Science 289:602-604 (28Jul. 2000), a scientific publication related to the 2002/0070426 A1patent publication, Cumings and Zettl describe a low friction nanoscalelinear bearing, which operates in a reciprocal (i.e. telescoping)manner.

Den et al. U.S. Pat. No. 6,628,053 discloses a carbon nanotube devicecomprising a support having a conductive surface and a carbon nanotube,wherein one terminus of the nanotube binds to the conductive surface sothat conduction between the surface and the carbon nanotube ismaintained. The device is used as an electron generator.

Falvo et al. Nature 397:236-238 (Jan. 21, 1997) disclose studiesinvolving the rolling of carbon nanotubes using atomic force microscope(AFM) manipulation of multiwall carbon nanotubes (MWCNT, termed in thepaper “CNT”). No bearing properties are disclosed.

Minett et al. Current Applied Physics 2:61-64 (2002) disclose the use ofcarbon nanotubes as actuators in which the driving force is obtainedfrom a deformation of the nanotube when a charge is applied. Theauthors, in their review also disclose the preparation of a suspendedcarbon nanotube across two metallic contacts growth of nanotubes acrosstwo metal contacts in a process that involved E-beam lithography andselective patterning.

Cumings et al. Nature 406:586 (Aug. 10, 2000) disclose techniques forpeeling and sharpening multiwall nanotubes. These sharpened tubes aredisclosed as having utility as biological electrodes, microscopic tips,etc.

Fraysse at al. Carbon 40:1735-1739 (2002) discloses carbon nanotubesthat act like actuators. In concept, a SWNT may be disposed above asubstrate and between a pair of metal-on-oxide layers. The nanotubes actas actuators through a cantilever effect achieved through longitudinaldeformation of the nanotube.

BRIEF SUMMARY OF THE INVENTION

The present invention provides a nanoscale actuator/motor device,comprising: (a) a substrate; (b) at least one anchor pad on or extendingfrom the substrate; (c) a nanotube mechanically and, optionally,electrically, connected to and extending from the anchor pad above thesubstrate to permit radial, i.e. rotational movement of an outer wallabout an inner wall of the nanotube, which movement may be either freelyrotating or torsional movement; (d) a rotor plate connected to an outersurface of the nanotube so as to move in connection with radial movementof the nanotube; and (e) at least one stator electrode disposed aboutthe rotor plate to electrically interact therewith when charged withsuitable voltage. The device may preferably comprise at least threestators disposed generally axially about the nanotube and close enoughto interact with the rotor plate. The substrate may preferably comprisea silicon chip etched to define an area of rotation between the rotorplate and the substrate. That is, a block of inert material such assilicon, which is crystalline and doped to be conductive, is providedwith a relatively insulating surface layer of SiO₂ upon whichelectrically isolated metal contacts may be formed. These contacts areused to create stators such as are known for use in electric motorshaving a rotor and stators. The stator electrodes preferably comprisetwo opposed stator electrodes disposed on opposite sides of themultiwalled carbon nanotube (MWNT) and rotor plate and a third stator inthe conductive silicon (e.g. single crystal or polycrystalline)substrate region. The opposed stator electrodes may further comprise aconductive material. Alternatively, the rotor plate or stators may bemade of magnetic material. The electrodes and rotor plate interact inthe illustrated device electrostatically; they may also interact withthe rotor plate magnetically. The rotor and the stator(s) “electricallyinteract” in a general sense that includes both magnetic andelectrostatic forces.

In a motor embodiment, the MWNT outer wall is separated from the anchorin a region between the rotor and the anchor to permit 360° of rotationof the MWNT outer wall relative to an inner wall. This can beaccomplished by torquing the tube until the material connecting theouter wall breaks free on either side of the rotor. It was found thatthis technique produces a nanotube which is freely rotatable about theinner MWNT walls, while the MWNT is fixed in place at the anchors

In operation, a voltage source is provided for delivering independentvoltages to the stators in a predetermined sequence to cause rotation ofthe MWNT by sequential interaction between the rotor plate andsuccessive stators. The voltage sources preferably comprise fourindependent voltage sources (or a single, four channel, voltage source),independently connected to the rotor plate and to the (preferably three)stator electrodes. The voltage source(s) may be alternated in oppositephases, in the case of the opposed stators, and in doubled frequency inthe case of a 90 degree offset gate stator, in order to cause rotatorplate to move fully through 360 degrees of rotation. Other sequences ofstator charge are illustrated in FIG. 4.

In manufacturing, gold with a chromium adhesion layer was applied to thenanotube and, incidentally, to the silicon substrate. The electrodes,stator and rotor were formed by a patterning, evaporation and lift offprocess using a combination of techniques that, individually, are knownin the semiconductor art. Thus, the substrate comprises a silicon oxidelayer coated with metal that is patterned to define the rotor and atleast one stator electrode. The stator electrodes comprise two opposedstator electrodes disposed on opposite sides of the rotor plate and athird stator on the surface below the rotor plate. Three dimensionalfeatures, i.e. anchor pads to secure the nanotube while permitting axialmovement and electrodes in different planes, are provided by etching theSiO₂ layer.

One aspect of the present invention is a method of fabricating ananoscale electric actuator/motor device having a nanotube attached to arotor plate, said nanotube suspended at either end between anchor pads,comprising: (a) providing a conductive silicon substrate covered with aless conductive layer (which may be silicon dioxide, quartz, mica,etc.); (b) depositing an MWNT on the less conductive layer; (c)depositing e-beam resist onto the tube and the less conductive layer;(d) lithographically patterning the e-beam resist; (e) depositing aconductive metal layer on the less conductive layer; and (f) etching thesubstrate around the MWNT to leave raised anchor pads and at least oneraised stator electrode covered with the conductive metal layer, with anetched away portion not covered with the conductive metal layer. Amicroscope such as an AFM or SEM is used to locate the MWNT's on thesubstrate and determine where to pattern the surrounding substrate inorder to coat the appropriate rotor plate areas and stators, and whereto etch away the silicon oxide contacting the nanotube and the rotorplate. Removal of silicon oxide is carried out by wet etching, such aswith hydrofluoric acid. Prior to etching, the electrode pattern isestablished with electron beam resist which is removed from selectiveareas by shining an e-beam on the desired areas and then chemicallyremoving the exposed resist with MIBK (methyl isobutyl ketone).

During the etching process that forms anchor pads and stators,undercutting caused by the fluid etchant removes material beneath therotor. It also may be used to cause a collapse of the anchor pads and/orstator(s), so as to allow positioning of the stators at differentpositions. This allows further radial distribution of the stators aboutthe nanotube. In this case device may have a stator electrode that is ona plane different from the rotor plate.

The present device is useful in higher order constructions, such asmicrofluidic devices, or electro-optic devices. In one such device, therotor plate is contacted with a fluid contained in a channel on thesubstrate in order to propel the fluid or to direct it into or block itfrom a channel on the opposite side of the rotor plate. Alternatively,the rotor plate may be illuminated with light to be modulated,redirected, deflected or reflected, by axial movement of the rotorplate. The rotation can be used to impart signal information to anextremely small light beam, to cause the light to scan, or for otherpurposes. The rotor can be easily adapted to a desired shape or size byaltering the lithographic process used to define it.

Finally, the present device may operate even at a significantly reducedatmospheric pressure, e.g. less than 10⁻⁵ torr.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective drawing of a rotational actuator/motor;

FIG. 2 is a top view electron micrograph of the rotationalactuator/motor of FIG. 1 prior to etching;

FIG. 3A-F is series of schematic drawings showing the presentfabrication process; and

FIG. 4 is a graph showing applied voltages and movement of the rotorplate in response thereto.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

I. Generalized Method and Apparatus

The present method and device utilizes a nanotube to which has beenaffixed a rotor plate that can be rotatably moved about the nanotubeaxis in a torsional, reciprocating manner (actuator) or, alternatively,can be rotated in a 360° spinning mode (motor). The axial movement isimparted by electrostatic forces between the rotor and at least onestator. These elements are electrically conductive and thereforegenerate electrical forces and fields that will cause movement of therotor through attractive or repulsive forces, either electrostaticallyor magnetically. Alternatively, if the rotor is made magnetic (e.g. ifthe rotor is made of a ferromagnetic material such as iron), it can beaccessed by magnetic fields. A spinning magnetic material generates anelectric current and may be used as a generator.

The preferred rotatable element is a multiwalled carbon nanotube (MWNT).These nanotubes have a near perfect carbon tubule structure thatresembles a sheet of sp² bonded carbon atoms rolled into a seamlesstube. They are generally produced by one of three techniques, namelyelectric arc discharge, laser ablation and chemical vapor deposition.The arc discharge technique involves the generation of an electric arcbetween two graphite electrodes, one of which is usually filled with acatalyst metal powder (e.g. iron, nickel, cobalt), in a Heliumatmosphere. The laser ablation method uses a laser to evaporate agraphite target which is usually filled with a catalyst metal powdertoo. The arc discharge and laser ablation techniques tend to produce anensemble of carbonaceous material which contains nanotubes (30-70%),amorphous carbon and carbon particles (usually closed-caged ones). Thenanotubes must then be extracted by some form of purification processbefore being manipulated into place for specific applications. Thechemical vapor deposition process utilizes nanoparticles of metalcatalyst to react with a hydrocarbon gas at temperatures of 500-900° C.A variant of this is plasma enhanced chemical vapor deposition in whichvertically aligned carbon nanotubes can easily be grown. In thesechemical vapor deposition processes, the catalyst decomposes thehydrocarbon gas to produce carbon and hydrogen. The carbon dissolvesinto the particle and precipitates out from its circumference as thecarbon nanotube. Thus, the catalyst acts as a ‘template’ from which thecarbon nanotube is formed, and by controlling the catalyst size andreaction time, one can easily tailor the nanotube diameter and lengthrespectively to suit. Carbon tubes, in contrast to a solid carbonfilament, will tend to form when the catalyst particle is ˜50 nm or lessbecause if a filament of graphitic sheets were to form, it would containan enormous percentage of ‘edge’ atoms in the structure. Alternatively,nanotubes may be prepared by catalytic pyrolysis of hydrocarbons asdescribed by Endo, et al., in J. Phys. Chem. Solids, 54, 1841 (1993), oras described by Terrones, et al., in Nature, 388, 52 (1997) or byKyotani, et al., in Chem. Mater., 8, 2190 (1996), the contents of all ofwhich are incorporated by reference.

Other forms of nanotube may be used, so long as they have uniformmechanical properties, have multiple wall layers and are mechanicallystable. Alternative forms of nanotubes (e.g. boron nitride) can beformulated with boron, nitrogen, or other elements; the key factors inselecting a nanotube are the nanoscale dimensions and the presence ofmultiple walls that can rotate relative to each other. Single wallednanotubes can also be used to provide a rotor support component forembodiments not involving free rotation, i.e. actuators, which havereciprocating radial movement.

The actuator/motor is essentially designed like an electric motor whichhas a plurality of electrically chargeable components in fixed relationto a rotating member on a nanotube axle. The overall size scale of thepresent actuator/motor is of the order of 300 nm. This will convey asense of the size of the present device, wherein the diameter of theMWNT is approximately 5 to 100 nanometers and the gap between the anchorpads can be as small as 200 nm.

As described in detail below, the components of the actuator/motor areintegrated on a silicon chip. That is, the stators are integral with thechip and the rotor plate is formed on the chip and when surroundingmaterial is etched away from the chip. Low-level externally appliedvoltages precisely control the operation speed and position of the rotorplate. Repeated oscillations of the rotor plate between positions 180°apart, as well as rotations of 360°, have been demonstrated with nosigns of wear or fatigue. Unlike existing chemically drivenbio-actuators and bio-motors, the present fully syntheticnanometer-scale electromechanical system (NEMS) actuator/motor isdesigned to operate over a wide range of frequency, temperature, andenvironmental conditions, including high vacuum and harsh chemicalenvironments.

FIG. 1 shows the conceptual design of the present device. The rotationalelement, rotor plate 10, a solid rectangular metal plate serving as arotor plate, is attached transversely to a suspended support shaft 12.The suspended support shaft 12 is a nanotube, preferably a multiwalledcarbon nanotube (MWNT) prepared as described above and deposited on asilicon substrate 14 having a silicon oxide layer 16 on top. The supportshaft ends 18, 20 are embedded in electrically conducting anchors (22,24) that rest on the oxidized surface 16 of silicon chip 14. The rotorplate assembly is surrounded by three fixed stator electrodes: two‘in-plane’ stators (S1, S2), 26, 28 are horizontally opposed and rest onunetched portions of the silicon oxide surface 16, and the third ‘gate’stator (S3) is buried beneath the etched surface (indicated at 30). Fourindependent (d.c. and/or appropriately phased a.c.) voltage signals, oneto the rotor plate and three to the stators (V1, V2, V3 and V4) can beapplied to control the position, speed and direction of rotation of therotor plate. The nanotube 12 serves simultaneously as the rotor plate 10support shaft and the electrical feed through to the rotor plate; mostimportantly it is also the source of rotational freedom.

It should be noted that surfaces shown as planar and perpendicular tothe top of the substrate are in fact curved, and undercut the topsurface. This occurs during the etching step, so that the conductivelayer 22 hangs over the substrate. That is, surfaces shown as verticalplanar surfaces in FIG. 1, such as surface 29, are in fact concave. Inthe case of the rotor plate 10, the substrate has been completelyundercut. This undercutting can also be used to create differences inheight as, e.g. between the stators 26, 28 and the anchors 22, 24, orbetween the two stators.

FIG. 2 shows a scanning electron micrograph illustrating anactuator/motor device prior to etching. While the components are notnumbered in the photograph, it is plain that they correspond to thestructures illustrated in FIG. 1. The scale bar in the lower leftportion of the scanning electron micrograph is 300 nm long andapproximates the rotor plate width (transverse to the support). Typicalrotor plate dimensions were 250-500 nm on a side.

II. Construction of the Device

A. MWNT's

MWNTs were synthesized by the standard arc technique as described inEbbesen et al. U.S. Pat. No. 5,641,466 issued Jun. 24, 1997, herebyincorporated by reference to describe a method for large-scale synthesisof carbon nanotubes. The technique that was used is also reported in theNature publication of reference (11). In an inert gas at a pressure of200-2500 torr, an arc discharge is made between two carbon rodelectrodes by application of a suitable AC or DC voltage (e.g. about 18V) to thereby produce a carbon plasma. The electric current is about50-100 A. As the result a carbon deposit forms on the end of one of thetwo carbon rods, and a core part of the carbon deposit contains a largeamount of carbon nanotubes. This core part can easily be separated froma shell part in which no carbon nanotubes exist. Usually carbonnanotubes occupy more than 65 wt % of the core part of the deposit, andthe nanotubes coexist with some (less than 35 wt %) carbon nanoparticleswhich are nanometer-scale carbon particles with polyhedral cagestructures. Sometimes a small amount of amorphous carbon also coexists.

B. Doped Silicon Substrate

The present device was formed by the deposition of various layers andcomponents onto a crystralline silicon chip. Degenerately doped siliconsubstrates were covered with 1 μm of thermally grown SiO₂. Pre-patternedalignment marks were placed on the substrate by standard lithographictechniques and were located a fixed distance apart for later reference.

The substrate is comprised of layers of silicon and silicon oxide. Inprocessing, materials are deposited for the formation of the electrodesand the rotor plate, as described and shown next in connection with FIG.3. Silicon was chosen because photolithographic, etching, and othertechniques for its manipulation are readily available. Other inertmaterials that can be physically shaped could also be used for thepresent actuator/motor, such as plastic polymer or glass. Material; suchas used in the resist could also be used

C. Deposition and Etching

The actuator/motor components (in-plane rotor plate, in-plane stators,anchors, and electrical leads) were then patterned in the substratecomprising the SiO₂ using electron beam lithography.

FIG. 3A-F represents an end view taken along line 3-3′ in FIG. 1. Forpurposes of illustration, a single MWNT is shown attached to thesubstrate, and the electrode behind the rotor plate is not shown.Referring now to FIG. 3A, the nanotube (e.g. an MWNT) 12 suspended in1,2-dichlorobenzene was deposited on the above-described substrate,comprising a silicon chip 16 coated with silicon oxide 16. The MWNT'swere located with respect to the pre-patterned alignment marks onsurface 16 using an atomic force microscope (AFM) or a LEO 1550 scanningelectron microscope (SEM). In this way, the subsequently described stepscould be accurately positioned around the selected nanotube(s).

As shown in FIG. 3B the MWNT 12 on the SiO₂ and the SiO₂ were coatedwith a layer of e-beam resist 32. In adding layer 32, a single layer ofelectron beam resist (polymethyl methacrylate, 950,000 relativemolecular mass, 5.5% in chlorobenzene) was spun on the substrate at4,000 r.p.m. for 45 seconds, and subsequently baked in air at 150° C.for 2 hours.

Next, as also shown in FIG. 3B, the resist was patterned usingcommercially available electron beam writing software, namely NPGSsoftware (Nanometer Pattern Generating System, which may be obtainedfrom available from Joe Nabity, Ph.D.JC Nabity Lithography Systems P.O.Box 5354 Bozeman, Mont. 59717 USA), loaded on a JEOL 6400 SEM (JEOL USA,Inc.). The JEOL-6400 with NPGS is a high-resolution, electron beamlithography system used for writing complex patterns in resists from thenanometer scale up to 5 mm. The striped regions in FIG. 3B representareas of resist where the e beam struck and disrupted the resist so thatit could be removed in subsequent steps. The electron beam resist wasdeveloped in methyl isobutyl ketone:isopropyl alcohol 1:3 for oneminute, causing removal of the resist, as shown in FIG. 3C.

Next, as shown in FIG. 3D, chromium (10 nm), then gold (90 nm) wasevaporated onto the nanotube and (incidentally) the surrounding area.The Cr layer improves adhesion of the gold that is used for electrodesand stators. Next, as shown in FIG. 3E, the resist that remained afterthe MIBK step (FIG. 3C), and the Au/Cr on top of it, were lifted off inacetone. The Cr/Au was subsequently annealed at 400° C. to ensure betterelectrical and mechanical contact between the Cr and the MWNT.

Then, as shown in FIG. 3F, an HF etch was used to remove roughly 500 nmof the SiO₂ surface 16 to provide clearance sufficient to permit therotor plate to be rotated by 90° (and more). Note that the area underthe rotor R is exposed to the HF from the sides through an undercuttingprocess so that the Au/CR attached to the nanotube is free of underlyingSiO₂. That is, in FIG. 3F, the tube and metal are resting on the anchors(not shown) that are into and above the plane of the drawing, along theaxis of the nanotube.

The conducting Si substrate (typically used as the gate electrode inthree-terminal nanotube field-effect devices (Refs. 12, 13) here servesas the gate stator, i.e. below the rotor plate.

III. Characterization and Operation

A. Torsional Spring (Actuator)

Initial actuator characterization was carried out in situ inside the LEOSEM. Applying voltages up to 50 V d.c. between the (slightly asymmetric)rotor plate and the gate stator (S3) generated a net torque sufficientto visibly rotate the rotor plate (up to 20° deflection). The rotorplate is slightly asymmetric in that one side extends further from theMWNT than the other. It should be noted that only one stator isnecessary to create a torsional spring.

When the applied voltage was removed, the rotor plate would rapidlyreturn to its original horizontal position. Using a finite analysisprogram (FEMLAB, a commercially available plug-in for MATLAB) and theactuator geometry together with the measured deflection and appliedvoltages, it was determined that a typical ‘as produced’ effectivetorsional spring constants was 10⁻¹⁵ to 10⁻¹² N m. Evaluation of theMWNT shear modulus (assuming a continuum mechanics model [Ref 14])necessitates knowledge of the outer radii of the nanotubes. The outerdiameter of the MWNTs in the present devices was determined to within20%. They ranged from 10 to 40 nm, which was consistent withhigh-resolution transmission electron microscopy (TEM) measurements ofMWNTs from the same preparation batch. TEM imaging also showed the MWNTsto be of high structural quality, composed of concentrically nestedcylindrical tubules with no obvious defects. A 10-nm-diameter MWNT withan effective length of 2 μm would have a shear modulus of 100 to 300GPa. These ranges for torsional spring constant and shear modulusoverlap those of more direct measurements employing a suspended MWNTsubjected to torsional deflection via an atomic force microscope tip(Refs 15, 16). Although the actuator/motor devices just described have anumber of extremely useful characteristics (including predictedtorsional oscillator mechanical resonance frequencies of the order oftens to hundreds of megahertz), the strong torsional spring constanteffectively prevents large low-frequency angular displacements. Thetorsional actuator/oscillator has significant applications that rely onthe resonance frequency described. For example, it can act as a bandpass filter to filter out certain frequencies going through the device.It can also be used to sense changes in fluid flow, when fluid iscontacted to the rotor and the displacement of the rotor is measured.Because the nanotube is stiff, it has a high resonance frequency and canbe useful in a variety of applications. The device will have a highquality factor, making them desirable as filters and for otherapplications utilizing the resonance frequency.

B. Rotator (Motor)

For large-displacement operation, including 360° rotation, the MWNTsupport shaft 12 (FIG. 1) was modified to exploit the intrinsiclow-friction bearing behavior afforded by the perfectly nested shells ofMWNTs (Refs. 9, 10, 17, 18). The modification comprises removing orcompromising one or more outer MWNT shells in the region between therotor plate 10 and the anchors 22, 24 (FIG. 1). Several in situ methodswere used to achieve the modification while the device was in place inthe in the LEO SEM, including reactive-ion etching, application ofcurrent through the nanotube to ‘blow out’ outer nanotube shells (Refs.19, 20), and selective nanotube bond-damage induced by the SEM electronbeam.

A particularly simple yet effective in situ MWNT modification method,and the one used on the devices to be described below, was tomechanically fatigue and eventually shear potions of the outer nanotubeshells (between the rotor and the anchor) by successive application ofvery large stator voltages. We found that applied gate stator voltagesof order 80 V d.c. would torque the outer nanotube shells past theelastic limit, eventually leading to partial or complete failure of theouter nanotube shells and a resulting dramatic increase in therotational freedom of the rotor plate. In the ‘free’ state, the rotorplate was still held in position axially by the intact nanotube coreshells, but could be azimuthally positioned, using an appropriatecombination of stator signals, to any arbitrary angle between 0° and360°. Once so positioned, the rotor plate nominally remained in placeeven with all stator voltages reduced to zero, eventually drifting to avertical (0° or 180°) position only under the charging influence of theSEM imaging electron beam.

Other methods to separate the outer shell and permit free rotation couldalso be used. For example, the outer shell of the MWNT could befractured using current passing through the nanotube. Or, a reactive ionetch could be used to break away outer walls.

In addition, partial breakage of an outer wall could be accomplishedusing these techniques. This would be useful in the actuator embodimentin that it would result in a weaker torsional spring. It could be usedto sense fluid flow or to achieve a lower resonance frequency. More thanone nanotube can be used together to support the rotor plate. In thiscase, the nanotubes can be damaged so that some of the tubes are nolonger intact.

To verify the operation of the device a series of still SEM images wererecorded of an actuator/motor device in the free state, being ‘walked’through one complete rotor plate revolution using quasi-static d.c.stator voltages. The stator voltages, never exceeding 5 V, were adjustedsequentially to attract the rotor plate edge to successive stators. Byreversing the stator voltage sequence, the rotor plate rotation could bereproducibly reversed. These images may be viewed in the correspondingpublication in Nature 424:408-410 (24 Jul. 2003) and accompanyingon-line materials. The images verify the rotation of the rotor plate asdescribed.

Finite frequency operation of the actuator/motor was also performed,using a variety of suitably phased a.c. and d.c. voltage signals to thethree stators and rotor plate. In one simple operation mode,out-of-phase common-frequency sinusoidal voltages were applied tostators S1, S2, and S3, and a d.c. offset to the rotor plate R; that is,S1=V₀ sin (ωt), S2=V₀ sin(ωt+240°), and S3=V₀ sin(ωt+120°), where ω is ½of the frequency of rotation, and R=−V₀. In this design, the stators areslightly below the plane of the nanotube/rotor/anchors. This dislocationof the stators (26, 28 FIG. 1) was accomplished by under etching thestators. This slight mismatch allows the simple voltage scheme describedhere to work, although other voltage schemes can be used in otherconfigurations. Using this drive sequence, one may reliably flip therotor plate between the extreme horizontal (90° and 270°) positions.Although in principle very high frequency operation should be possible(restricted only by the stripline bandwidth of the leads and,ultimately, inertial effects of the rotor plate), our SEM image capturerate limited direct real-time observations of rotor plate oscillationsto frequencies of typically several hertz.

Referring now to FIG. 4, suitable phased ac signals S1, S2, S3 inrelation to the position of the rotor 10 are shown. The position of therotor, as shown in the bottom row, changes relative to the combinedeffects of the voltages S1, S2 and S3, which follow the formulasdescribed above. Standard voltage sources are used. Other configurationscan be designed based on this example. It is simply necessary to placethe stators in an arrangement so that they can affect the rotor in arange of rotational positions. In this case, when S1 is at a peak and S2and S3 are between zero and a negative peak, the rotor is approximatelyat the default horizontal position (90°). At the peak positive S3 andnear peak negative S2 and S3 voltages (shown at 40), the rotor hasrotated 90° to the 180° position. When the S3 voltage returns toapproximately ⅜ negative peak (shown at 42), the rotor, as illustrated,has advanced to the next horizontal position, 270°. At the peak positiveS3 and positive S2 voltages, and near-negative S1 and S3 (shown at 44)voltages, the rotor has advanced to the 0° position and proceed fromthere to the 90° position, etc

The transitions as described above (between the extreme horizontalpositions) were recorded in digital video of the SEM images. Therecording captured an a.c. voltage driven actuator/motor ‘flipping’between the extreme horizontal positions (90° and 270°) in 33milliseconds. Video samples are available in the corresponding on linepublication corresponding publication in Nature 424:408-410 (24 Jul.2003).

The transitions between positions could be made faster than the imagevideo capture rate of 33 ms. Two images of the actuator/motor, recorded33 ms apart, showed the rotor plate respectively in the 90° and 270°positions. Actuator/motors were rotationally driven in this fashion formany thousands of cycles, with no apparent wear or degradation inperformance. In this configuration, the MWNT clearly serves as areliable, presumably wear-free, NEMS element providing rotationalfreedom. This characterization was performed in a pressure of 10⁻⁶-10⁻⁵torr, although reliable operation at higher pressures is anticipated.

The present actuator/motor may be characterized as the first trueMWNT-based NEMS device, in that it fully integrates electronic controland mechanical response. This distinguishes it from previous relatedMWNT-based mechanical devices which require relatively large and complexexternal control systems (such as piezo-driven manipulators) to achieveoperation 15-18, 21.

IV. Applications

The present disclosure suggests that the present nanotube-basedactuator/motors have a number of MEMS/NEMS applications. The rotorplate, when covered with metal, could serve as a mirror, with obviousrelevance to ultra-high-density optical sweeping and switching devices(the total actuator/motor size is just at the limit of visible lightfocusing). In this case, a light source would be directed onto the rotorR from a position above the substrate. The light source could be anytype of optical signal. The rotor plate could also serve as a paddle forinducing and/or detecting fluid motion in microfluidics systems, as agated catalyst in wet chemistry reactions, as a bio-mechanical elementin biological systems, or as a general (potentially chemicallyfunctionalized) sensor element. In a microfluidics application, thefluid would be channeled between an actuator and an anchor, and suchprojections would be etched in a way so as to define fluid impermeablechannels. It is also possible that the charged oscillating metallicplate could be used as a transmitter of electromagnetic radiation.

While the foregoing device and its method of construction and operationhas been described in reference to particular embodiments, manyvariations and embellishments are possible in view of the aboveteachings. Therefore, it is intended that the present invention not belimited to the specific embodiments described above, but rather to thescope of the appended claims.

REFERENCES

-   1. Tour, J. M. et al. Recent advances in molecular scale    electronics. Ann. NY Acad. Sci. 852, 197-204 (1998).-   2. Judy, J. W. Microelectromechanical system (MEMS): Fabrication,    design and applications. Smart Mater. Struct. 10, 1115-1134 (2001).-   3. Craighead, H. G. Nanoelectromechanical systems. Science 290,    1532-1535 (2000).-   4. Roukes, M. L. in Tech. Digest of the 2000 Solid-State Sensor and    Actuator Workshop (eds Bousse, L. & Schmidt, M.) 367-376 (Transducer    Research Foundation, Cleveland, 2000)-   5. Carr, D. W., Evoy, S., Sekaric, L., Craighead, H. G. &    Parpia, J. M. Measurement of mechanical resonance and losses in    nanometer scale silicon wires. Appl. Phys. Lett. 75, 920-922 (1999).-   6. Lifshitz, R. & Roukes, M. L. Thermoelastic damping in micro- and    nanomechanical systems. Phys. Rev. B 61, 5600-5609 (2000).-   7. Iijima, S. Helical microtubules of graphitic carbon. Nature 354,    56-58 (1991).-   8. Chopra, N. G. et al. Boron nitride nanotubes. Science 269,    966-967 (1995).-   9. Charlier, J.-C. & Michenaud, J.-P. Energetics of multilayered    carbon tubules. Phys. Rev. Lett. 70, 1858-1861 (1993).-   10. Kolmogorov, A. N. & Crespi, V. H. Smoothest bearings: Interlayer    sliding in multiwalled carbon nanotubes. Phys. Rev. Lett. 85,    4727-4730 (2000).-   11. Ebbesen, T. W. & Ajayan, P. M. Large-scale synthesis of carbon    nanotubes. Nature 358, 220-222 (1992).-   12. Tans, S. J. et al. Individual single-wall carbon nanotubes as    quantum wires. Nature 386, 474-477 (1997).-   13. Bockrath, M. et al. Single electron transport in ropes of carbon    nanotubes. Science 275, 1922-1925 (1997).-   14. Yakobson, B. I., Brabec, C. J. & Bemholc, J. Nanomechanics of    carbon tubes: Instabilities beyond linear response. Phys. Rev. Lett.    76, 2511-2514 (1996).-   15. Williams, P. A. et al. Torsional response and stiffening of    individual multiwalled carbon nanotubes. Phys. Rev. Lett. 89, 255202    (2002).-   16. Williams, P. A. et al. Fabrication of nanometer-scale mechanical    devices incorporating multiwalled carbon nanotubes as torsional    springs. Appl. Phys. Lett. 82, 805-807 (2003).-   17. Cumings, J. & Zettl, A. Low-friction nanoscale linear bearing    realized from multi-walled carbon nanotubes. Science 289, 602-604    (2000).-   18. Yu, M.-F., Yakobson, B. I. & Ruoff, R. S. Controlled sliding and    pullout of nested shells in individual multiwalled carbon    nanotubes. J. Phys. Chem. B 104, 8764-8767 (2000).-   19. Cumings, J., Collins, P. G. & Zettl, A. Peeling and sharpening    of multiwall nanotubes. Nature 406, 586 (2000).-   20. Collins, P. G., Arnold, M. S. & Avouris, P. Engineering carbon    nanotubes and nanotube circuits using electrical breakdown. Science    292, 706-709 (2001).-   21. Poncharal, P., Wang, Z. L., Ugarte, D. & de Heer, W. A.    Electrostatic deflections and electromechanical resonances of carbon    nanotubes. Science 283, 1513-1516 (1999).

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 24. A method ofrotating a nanotube attached to a rotor plate, said nanotube suspendedat opposed portions between anchor pads on a substrate to permitrotational movement of the rotor plate without hindrance by thesubstrate, comprising: (a) providing at least two stator electrodesadjacent the rotor plate; (b) applying out of phase voltages to thestator electrodes; while (c) applying a dc voltage offset to the statorvoltages to the rotor plate.
 25. The method of claim 24 wherein thesteps of providing stator electrodes and applying voltages furthercomprise: (a) providing two stator electrode S1 and S2 on opposite sidesof the nanotube and a stator electrode S3 approximately orthogonal to aline between S1 and S2; and (b) applying out of phase, common frequencyalternating voltages to stator electrodes S1 and S2.
 26. The method ofclaim 25 further comprising the step of: (c) applying a double frequencysignal to electrode S3.
 27. The method of claim 24 further comprisingthe step of contacting the rotor plate with a fluid contained in achannel on the substrate.
 28. The method of claim 24 further comprisingthe step of contacting the rotor plate with light to be modulated byrotational movement of the rotor plate.
 29. The method of claim 24further comprising the step of contacting the rotor plate with light tobe deflected by rotational movement of the rotor plate.
 30. The methodof claim 24 further comprising the step of reciprocally rotating therotor plate at a resonance frequency determined by stiffness of thenanotube.
 31. The method of claim 30 further comprising the step ofmodifying the stiffness of the nanotube by breaking portions of thenanotube.
 32. The method of claim 24 further comprising: (a) providingthe substrate covered with a less conductive layer; (b) depositing thenanotube attached to the rotor plate on the less conductive layer; (c)depositing a conductive metal layer on the less conductive layer; and(d) patterning the substrate around the nanotube attached to the rotorplate to leave raised anchor pads and at least one raised statorelectrode covered with the conductive metal layer, with an etched awayportion not covered with the conductive metal layer.
 33. The method ofclaim 32 wherein the nanotube is selected from the group consisting ofsingle walled carbon nanotubes, multiwalled carbon nanotubes and boronnitride nanotubes.
 34. The method of claim 32 further comprising thestep of breaking portions of the nanotube, thereby changing thestiffness of the nanotube.
 35. The method of claim 32 further comprisingthe step of breaking portions of the nanotube, thereby permitting freerotation of the rotor plate.
 36. The method of claim 32 comprising thestep of: breaking portions of a nanotube to permit greater torsionalmovement.
 37. The method of claim 32 wherein said patterning comprisesremoving silicon oxide patterned on the substrate, carried out by wetetching.
 38. The method of claim 37 wherein said patterning furthercomprises treating a coating of electron beam resist with an electronbeam.
 39. A method of rotating a nanoscale device, comprising: (a)providing a substrate; (b) providing a freely rotating nanoscale deviceattached to the substrate; (c) a means for rotating the freely rotatingnanoscale device.
 40. The method of claim 39 further comprising the stepof contacting the nanoscale device with light to be deflected byrotational movement of the nanoscale device.
 41. The method of claim 39further comprising the step of contacting the nanoscale device with afluid contained in a channel on the substrate.
 42. The method of claim39 further comprising: (a) means for fabricating the nanoscale device.